Brain diseases such as cerebrovascular disease, depression, and neurodegenerative disorder such as Alzheimer's disease are caused by a weakening of or damage to the central nervous system cells. In order for these brain diseases to be treated, research is being conducted on regenerative therapies of the brain in which damaged neurocytes are protected by transplantation of new cells into the brain or injection of a neurotrophic factor into the brain. Although the majority of these regenerative therapies are still under study, some are already being applied clinically, and thus are receiving attention as new therapeutic treatments for various brain diseases. For example, Non-Patent Documents 1 and 2 disclose a therapeutic treatment for introducing a neurotrophic factor into the brain, by transplanting cells producing a neurotrophic factor into the brain to compensate for deficient amounts of the neurotrophic factor.    Non-Patent Document 1: “Regenerative Treatments of the Brain (Nou-no-saisei-iryou),” (online), Japan Neurosurgical Society, http://square.umin.ac.jp/neuroinf/patient/701.html (accessed Oct. 16, 2006); and    Non-Patent Document 2: “Perspective on Neuroprotective and Neural Repairing Agents (Shinkei-hogo, shinkei-shuhukuyaku-no-tenbou),” (online), http://www.h2.dion ne.jp/˜park/index1/i1014hogo.html (accessed Oct. 16, 2006).